The science behind sunscreen application
The FDA requires sunscreens to be tested at 2mg/cm², which means two milligrams of sunscreen per square centimeter of skin. "It's pretty hard to measure how much skin you have on your face, which is why they say 1/2 teaspoon for your face and neck, I encourage everyone at least once in their life to actually measure out the exact amount one day, just to see how much that actually is. It'll feel like a lot if you're not used to it, so We recommend a two step application process. Apply a little and wait a few minutes...then apply the rest.
Makeup is never enough
Well sort of. If your foundation has sunscreen in it, you still have to abide the 2mg/cm² rule, but it's "close to impossible to wear that much foundation." (You would have to pump out at least 11 pumps from a standard cosmetic dispenser) Your best bet is to find a sunscreen that works as a nice base under makeup. Apply enough, let it set, and then go forth with your makeup while trying not to disturb the sunscreen too much.